Remember that the optimal distances between the antennas are calculated and used in order to get correct phasing on the RF signals. The cable lengths must be a part of this equation in order to achieve optimal distance. So don’t cut the original cables – keep the same cable lengths on all antennas – or do some more advanced calculations.
A short rule is that the distance between the antennas should be a multiple of half the wavelength.
For 11n that uses MIMO you should use three equal antennas for each band covering the same area. You should not try to extend the coverage area by separating the antennas too much or i.e. use three sector antennas in different or slightly different directions.
Your gut feeling about distance between 2,4GHz antennas and 5GHz antennas is good. You should have some physical distance but since there is 2,6GHz between these bands the inbuilt filters should give enough separation.
